Python: Files and Exceptions

• Focus of the text is files and exceptions in Python programming.
• It covers file input/output, processing files with loops, handling records, and managing exceptions.

Introduction to File Input and Output

• Data must be saved to a file to persist between program runs, typically on a computer disk.
• Saved data can be retrieved and used later.
• "Writing data to" means saving data to a file.
• An output file is a file that data is written to.
• "Reading data from" refers to retrieving data from a file.
• An input file is a file from which data is read.
• Three steps for using a file: open, process, and close.

Types of Files and File Access Methods

• Text files contain data encoded as text.
• Binary files contain data not converted to text.
• Sequential access reads files from beginning to end, without skipping.
• Direct access allows jumping to specific data within a file.

Filenames and File Objects

• Filename extensions are short character sequences at the end of a filename, preceded by a period, indicating the data type of the file.
• File objects are associated with specific files and provide a way for a program to interact with the file.
• A file object is referenced by a variable.

Opening a File

• The open function creates a file object and associates it with a disk file.
• The general format is file_object = open(filename, mode).
• Mode is a string specifying how the file opens, such as reading ('r'), writing ('w'), or appending ('a').
• outfile = open('philosophers.txt', 'w') is an example of opening a file named "philosophers.txt" for writing.
• Without a path, Python assumes the file is in the same directory as the program.
• You can specify an alternative path and file name in the open function argument, prefixed with 'r'.

Writing Data to a File

• A method is a function belonging to an object that performs operations on that object.
• The file object's write method writes data to the file.
• file_variable.write(string) illustrates the write method's format.
• Close a file using the file object's close method, file_variable.close().

Reading Data From a File

• The read method reads the entire file content into memory, working only if the file opens for reading.
• The content is returned as a string.
• infile = open('philosophers.txt', 'r') is an example of opening a file for reading.
• The readline method reads a single line from the file, including the newline character (\n), and returns it as a string.
• Read position marks where the next item will be read from in the file.

Concatenating a Newline and Stripping It

• Concatenating a newline character (\n) is often necessary before writing data to a file, using the + operator.
• The rstrip method, such as line1 = line1.rstrip('\n'), removes specific characters (like \n) from the end of a string after it is read from a file.

Appending Data to an Existing File

• Use the 'w' mode to overwrite existing files.
• Use the 'a' mode to append data to a file.
• If the file does not exist, the 'a' mode creates it; otherwise, it adds to the end of the file without erasing existing content.

Writing and Reading Numeric Data

• Numbers need converting to strings before writing to a file.
• The str function converts a value to a string.
• Numbers are read as strings from text file.
• Use int and float functions to convert strings to numeric values for mathematical operations.

Using Loops to Process Files

• Files often store large data amounts.
• Loops are common for reading from and writing to files.
• The readline method uses an empty string as a sentinel when the end of the file reaches.
• A while loop can be structured with the condition while line != '':

Python's while Loop to Read Lines

• The while loop format is while line != '': followed by statements, iterating through each line until an empty string returns.

Python's for Loop to Read Lines

• Python for loops automatically read lines and stop at the end of a file.
• The for loop format is for line in file_object: followed by statements, iterating once for each line.

Processing Records

• Records are frequently organized data within a file, with one record comprising a complete data set, and a field as an individual piece of data within that record.
• Records can be writing sequentially one field after another.
• You can read a record sequentially field by field to compelte the record.
• Use commas to separate data intead of using "new line", this creates the format known as CSV (comma separated values).
• part_no, part_name, the_rest = record.split(',', 2) is an exmaple extracting just the first two fields, the rest goes into variable "the_rest".
• part_no, part_name, make, model, quantity, price = record.split(',') Is a way to split a record into it's fields.

Processing CSV Files

• It is possible to split a CSV file by removing the coma by using record.split(',')
• Convert strings from CSV files to numeric formats using float() or int().
• This can be used to extract just the first 2 flelds, and include the rest.

Adding a CSV Record

• The code new_item = f'{part_no},{part_name},{make},{model},{quantity},{price}\n' creates a new record in CSV format using an f-string.
• Open a file to accept new information by using parts_file = open('parts.csv','a')

Operating System Commands for Deleting and Updating Records

• A temprary file can be created and used to update the information when deleting and updating record entries.
• The following are examples of commands available that can be used:
• import os
• os.remove('parts.csv')
• os.rename('temp.csv', 'parts.csv')

Exceptions

• Exceptions are errors during program execution, like division by zero, often halting the program.
• A traceback is an error message detailing line numbers and the type of error, such as ZeroDivisionError or ValueError.

Handlers and Suites

• Exception handlers are codes that respond when exceptions raised and prevent program crashing.
• They use the try/except statement.
• The try suite contains statements that might raise an exception.
• Handlers are statements within the except block.
• Executing a program after it raises an exception requires the except clause.

Exceptions - Handling

• Handlers skip if there is no exceptions raised.
• Create validations for user inputs.
• Converting inputs to a non-numeric string using an integer will result in error.

Handling Multiple Exceptions

• Code in the try suite can cause multiple exception types.
• Separate except clauses should handle each specific exception type.
• An except clause without specifying an exception type handles any unspecified exception.
• It should always be the last one.

Displaying the Default Error Message

• Print the exception object variable to display the defualt error message.

else Clause

• The try/except statement can contain an else clause following all except clauses.
• It contains the suite which get excuted after the try suite statements, only if no expection was raised.
• The else suite is skipped if there are raised excpetions.

finally Clause

• The try/except statement can contain finally clause after all except clauses.
• The finally suite statements exectutes whether or not a expection occurs in the code.
• Its purpose is to always preform a cleanup before exiting.

Unhandled Exceptions

• Exceptions go unhandled when no except clause defines them, or they raise outside a try suite.
• The above will cause all exceptions to halt the program.
• Use Phyton documentations to find different function expections.

With statement in Python

• A with statment should be implemented to make the code less complex and enhance exception handling.
• Automate the closing of the file.

With statment in Usage

• the format for opening a with statement is : with open('file.txt',r) as f:

Final Note

• A nested with statement, the interpreter closes the file automatically.